MANSOURA, Egypt, Jan 14 (Reuters) - As Egyptians vote in a constitutional referendum meant to help bring stability, policemen such as Brigadier-General Sayed Emara have good reason to dig in for more bloodshed.

He lost colleagues and friends last month, when a suicide bomber in a car ripped open a five-story building where security officials in the Nile Delta city of Mansoura worked. Seventeen people, most of them police, were killed.

The next day, the government declared the Muslim Brotherhood a terrorist group, encouraging policemen, who are frequently targeted, to harden their positions against the movement which says the army and security forces robbed it of power.

"It's not a war out in the open, that you can fight straight on, no, it comes after you in your house when you are with your family, or in your car or while you are walking on the street or working in your office," Emara told Reuters.

While much of the world's attention has focused on a security crackdown on Islamists in the struggle between the Brotherhood and the army-backed government, killing and detaining thousands, Egypt's police have also paid a high price.

At least 250 of them have been killed in militant attacks since army chief General Abdel Fattah al-Sisi deposed the elected Islamist president, Mohamed Mursi, in July - provoking a public reaction almost inconceivable just three years ago.

In the 1990s, another period when Islamist militants were attacking security forces, Egyptians would have had little sympathy for the police, often accused of corruption and brutality.

Security forces and police were put on the defensive after the popular uprising that toppled autocrat Hosni Mubarak in 2011. Many simply took off their uniforms and vanished.

Now they and Interior Ministry officials have manoeuvered their way back to power and are now lionised by the public. Fallen policemen are treated like heroes. Their posters hang in towns and villages.

The new charter strips out Islamist language and strengthens the state institutions that defied Mursi: the military, the police and the judiciary.

A big turnout and overwhelming "yes" vote would bolster the status of the police by making clear Egyptians are in favour of the new political order they are part of.

CONSPIRACY NARRATIVE

But the sense of siege officers feel in Mansoura and elsewhere shows no sign of easing and the adulation does not help ease the nerves of policemen or boost their poor social or economic standing.

Islamist militants from the Sinai Peninsula have stepped up attacks against police and soldiers. The violence has spread to cities, including Cairo, where the interior minister survived an assassination attempt.

Yet the average policeman earns about 1,800 Egyptian pounds ($258) - a low salary compared to others who serve the state. After Mubarak's downfall, policemen went on strike to demand better pay.

The Mansoura blast, one of the biggest attacks since Mursi's fall, rattled the typically gritty Nile Delta city of 500,000 and left policemen feeling more vulnerable.

"I didn't see the car approach the entrance of our building. All I heard was the huge explosion," said Ahmed Mathar, a recent graduate of the four-year programme at Cairo's Police Academy, who suffered wounds from flying shards of glass.

Mansoura's police now work in a building borrowed from a state-owned fertiliser company, their offices wrecked.

They often discuss ways of confronting Islamist militancy as plain-clothes security with pistols tucked into their belts hover around.

Emara, a slick 30-year veteran of the Interior Ministry who was wearing a charcoal grey suit, barks orders down the phone.

While the country focuses on the referendum, Emara tuned in to a television programme on the Palestinian militant group Hamas, an offshoot of the Muslim Brotherhood which controls the neighbouring Gaza Strip.

Like many Egyptian security officials, he is convinced Hamas is funding and providing weapons to al Qaeda-inspired militants in the Sinai which plot against the state. Hamas denies the allegations. The Brotherhood says it is a peaceful movement.

Even though Egypt has crushed the Brotherhood, killing hundreds of supporters and arresting its leaders, the group is still portrayed as highly dangerous.

It is a narrative that has had a major impact on the psyche of policemen in Mansoura and other parts of Egypt, encouraging them to harden their positions.

"We are working against a terrorist organisation that has many friends outside who want to destroy Egypt," said Emara, 52. "People must understand that the crimes of terrorism necessitate a severe response."

Hardline approaches are likely to invite more violence from Islamist militants, and more death in the police force.

The village of Bedawi, not far from Mansoura, is well aware of that reality. One of its sons, 33-year-old policeman Amr Hassan el-Tehan, was killed during the Mansoura bombing as he monitored communications equipment on the late shift.

"The Muslim Brotherhood. Hamas. Al Qaeda. Why are they like this?" asked his older brother Attiya. (Editing by Michael Georgy and Alison Williams)